PROJECT SUMMARY/ABSTRACT PTEN and FMR1 are two susceptibility genes for autism spectrum disorder (ASD) that encode regulators of the PI3K-Akt-mTOR pathway. Phosphorylation of ribosomal protein S6 (p-S6) is a downstream readout of mTOR activity. Altered levels of p-S6 have been reported in the postmortem cerebral cortex of individuals with autism and in mouse models of autism risk factors. However, it is not known when during development and in which cell types dysregulation of p-S6 signaling occurs and whether this contributes to the symptoms of ASD. Our goal is to identify common cell types and time windows in which p-S6 is dysregulated across two mouse models of autism risk factors, Pten and Fmr1, and to study the relationship between p-S6 dysregulation and social behavioral deficits. The novel hypothesis we develop here is that cell types that are normally enriched for p-S6 in the developing brain are selectively vulnerable to overgrowth caused by Pten or Fmr1 mutations. We propose to carry out this work by using an innovative combination of mapping the activity of a signaling pathway (PI3K-Akt-mTOR/p-S6) key to cellular growth and ASD pathogenesis in the developing brain at unprecedented spatiotemporal resolution, neuroanatomical tracing and behavioral phenotyping to determine when and where dysregulation of p-S6 might contribute to the pathophysiology of ASD risk factors. In addition to understanding molecular and cellular mechanism of social behavioral deficits, this will also test the therapeutic potential of targeting mTOR/p-S6 signaling during a critical developmental time window as a strategy for treating ASD.